Binding of bivalent ions to actinomycete mannanase is accompanied by conformational change and is a key factor in its thermal stability

Impact of Modular Architecture on Activity of Glycoside Hydrolase Family 5 Subfamily 8 Mannanases

Glycoside hydrolase family 5 subfamily 8 (GH5_8) mannanases belong to Firmicutes, Actinomycetia, and Proteobacteria. The presence or absence of carbohydrate-binding modules (CBMs) present a striking difference. While various GH5_8 mannanases need a CBM for binding galactomannans, removal of the CBM did not affect activity of some, whereas it in other cases reduced the catalytic efficiency due to increased K_M. Here, monomodular GH5_8 mannanases from Eubacterium siraeum (EsGH5_8) and Xanthomonas citri pv. aurantifolii (XcGH5_8) were produced and characterized to clarify if GH5_8 mannanases from Firmicutes and Proteobacteria without CBM(s) possess distinct properties. EsGH5_8 showed a remarkably high temperature optimum of 55 °C, while XcGH5_8 had an optimum at 30 °C. Both enzymes were highly active on carob galactomannan and konjac glucomannan. Notably, EsGH5_8 was equally active on both substrates, whereas XcGH5_8 preferred galactomannan. The K_M values were comparable with those of catalytic domains of truncated GH5_8s, while the turn-over numbers (k_cat) were in the higher end. Notably, XcGH5_8 bound to but did not degrade insoluble ivory nut mannan. The findings support the hypothesis that GH5_8 mannanases with CBMs target insoluble mannans found in plant cell walls and seeds, while monomodular GH5_8 members have soluble mannans and mannooligosaccharides as primary substrates.

Functional diversity of three tandem C-terminal carbohydrate-binding modules of a β-mannanase

Carbohydrate active enzymes, such as those involved in plant cell wall and storage polysaccharide biosynthesis and deconstruction, often contain repeating noncatalytic carbohydrate-binding modules (CBMs) to compensate for low-affinity binding typical of protein–carbohydrate interactions. The bacterium Saccharophagus degradans produces an endo-β-mannanase of glycoside hydrolase family 5 subfamily 8 with three phylogenetically distinct family 10 CBMs located C-terminally from the catalytic domain (SdGH5_8-CBM10x3). However, the functional roles and cooperativity of these CBM domains in polysaccharide binding are not clear. To learn more, we studied the full-length enzyme, three stepwise CBM family 10 (CBM10) truncations, and GFP fusions of the individual CBM10s and all three domains together by pull-down assays, affinity gel electrophoresis, and activity assays. Only the C-terminal CBM10-3 was found to bind strongly to microcrystalline cellulose (dissociation constant, K_d = 1.48 μM). CBM10-3 and CBM10-2 bound galactomannan with similar affinity (K_d = 0.2–0.4 mg/ml), but CBM10-1 had 20-fold lower affinity for this substrate. CBM10 truncations barely affected specific activity on carob galactomannan and konjac glucomannan. Full-length SdGH5_8-CBM10x3 was twofold more active on the highly galactose-decorated viscous guar gum galactomannan and crystalline ivory nut mannan at high enzyme concentrations, but the specific activity was fourfold to ninefold reduced at low enzyme and substrate concentrations compared with the enzyme lacking CBM10-2 and CBM10-3. Comparison of activity and binding data for the different enzyme forms indicates unproductive and productive polysaccharide binding to occur. We conclude that the C-terminal-most CBM10-3 secures firm binding, with contribution from CBM10-2, which with CBM10-1 also provides spatial flexibility.

Mannan endo-1,4-β-mannosidase from Kitasatospora sp. isolated in Indonesia and its potential for production of mannooligosaccharides from mannan polymers

Mannan endo-1,4-β-mannosidase (commonly known as β-mannanase) catalyzes a random cleavage of the β-D-1,4-mannopyranosyl linkage in mannan polymers. The enzyme has been utilized in biofuel production from lignocellulose biomass, as well as in production of mannooligosaccharides (MOS) for applications in feed and food industries. We aimed to obtain a β-mannanase, for such mannan polymer utilization, from actinomycetes strains isolated in Indonesia. Strains exhibiting high mannanase activity were screened, and one strain belonging to the genus Kitasatospora was selected. We obtained a β-mannanase from this strain, and an amino acid sequence of this Kitasatospora β-mannanase showed a 58-71% similarity with the amino acid sequences of Streptomyces β-mannanases. The Kitasatospora β-mannanase showed a significant level of activity (944 U/mg) against locust bean gum (0.5% w/v) and a potential for oligosaccharide production from various mannan polymers. The β-mannanase might be beneficial particularly in the enzymatic production of MOS for applications of mannan utilization.

Characterization of pH-tolerant and thermostable GH 134 β-1,4-mannanase SsGH134 possessing carbohydrate binding module 10 from Streptomyces sp. NRRL B-24484

A GH 134 β-1,4-mannanase SsGH134 from Streptomyces sp. NRRL B-24484 possesses a carbohydrate binding module (CBM) 10 and a glycoside hydrolase 134 domain at the N- and C-terminal regions, respectively. Recombinant SsGH134 expressed in Escherichia coli. SsGH134 was maximally active within a pH range of 4.0-6.5 and retained >80% of this maximum after 90 min at 30°C within a pH range of 3.0-10.0. The β-1,4-mannanase activity of SsGH134 towards glucomannan was 30% of the maximal activity after an incubation at 100°C for 120 min, indicating that SsGH134 is pH-tolerant and thermostable β-1,4-mannanase. SsGH134, SsGH134-ΔCBM10 (CBM10-linker-truncated SsGH134) and SsGH134-G34W (substitution of Gly34 to Trp) bound to microcrystalline cellulose, β-mannan and chitin, regardless of the presence or absence of CBM10. These indicate that GH 134 domain strongly bind to the polysaccharides. Although deleting CBM10 increased the catalytic efficiency of the β-1,4-mannanase, its disruption decreased the pH, solvent and detergent stability of SsGH134. These findings indicate that CBM10 inhibits the β-1,4-mannanase activity of SsGH134, but it is involved in stabilizing its enzymatic activity within a neutral-to-alkaline pH range, and in the presence of various organic solvents and detergents. We believe that SsGH134 could be useful to a diverse range of industries.

Mannoside recognition and degradation by bacteria

Mannosides constitute a vast group of glycans widely distributed in nature. Produced by almost all organisms, these carbohydrates are involved in numerous cellular processes, such as cell structuration, protein maturation and signalling, mediation of protein-protein interactions and cell recognition. The ubiquitous presence of mannosides in the environment means they are a reliable source of carbon and energy for bacteria, which have developed complex strategies to harvest them. This review focuses on the various mannosides that can be found in nature and details their structure. It underlines their involvement in cellular interactions and finally describes the latest discoveries regarding the catalytic machinery and metabolic pathways that bacteria have developed to metabolize them.

Biochemical characterization of the novel endo-β-mannanase AtMan5-2 from Arabidopsis thaliana

Plant mannanases are enzymes that carry out fundamentally important functions in cell wall metabolism during plant growth and development by digesting manno-polysaccharides. In this work, the Arabidopsis mannanase 5-2 (AtMan5-2) from a previously uncharacterized subclade of glycoside hydrolase family 5 subfamily 7 (GH5_7) has been heterologously produced in Pichia pastoris. Purified recombinant AtMan5-2 is a glycosylated protein with an apparent molecular mass of 50kDa, a pH optimum of 5.5-6.0 and a temperature optimum of 25°C. The enzyme exhibits high substrate affinity and catalytic efficiency on mannan substrates with main chains containing both glucose and mannose units such as konjac glucomannan and spruce galactoglucomannan. Product analysis of manno-oligosaccharide hydrolysis shows that AtMan5-2 requires at least six substrate-binding subsites. No transglycosylation activity for the recombinant enzyme was detected in the present study. Our results demonstrate diversification of catalytic function among members in the Arabidopsis GH5_7 subfamily.

The GH5 1,4-β-mannanase from Bifidobacterium animalis subsp. lactis Bl-04 possesses a low-affinity mannan-binding module and highlights the diversity of mannanolytic enzymes

Background

β-Mannans are abundant and diverse plant structural and storage polysaccharides. Certain human gut microbiota members including health-promoting Bifidobacterium spp. catabolize dietary mannans. Little insight is available on the enzymology of mannan deconstruction in the gut ecological niche. Here, we report the biochemical properties of the first family 5 subfamily 8 glycoside hydrolase (GH5_8) mannanase from the probiotic bacterium Bifidobacterium animalis subsp. lactis Bl-04 (BlMan5_8).

Results

BlMan5_8 possesses a novel low affinity carbohydrate binding module (CBM) specific for soluble mannan and displays the highest catalytic efficiency reported to date for a GH5 mannanase owing to a very high k_cat (1828 ± 87 s^-1) and a low K_m (1.58 ± 0.23 g · L^-1) using locust bean galactomannan as substrate. The novel CBM of BlMan5_8 mediates increased binding to soluble mannan based on affinity electrophoresis. Surface plasmon resonance analysis confirmed the binding of the CBM10 to manno-oligosaccharides, albeit with slightly lower affinity than the catalytic module of the enzyme. This is the first example of a low-affinity mannan-specific CBM, which forms a subfamily of CBM10 together with close homologs present only in mannanases. Members of this new subfamily lack an aromatic residue mediating binding to insoluble cellulose in canonical CBM10 members consistent with the observed low mannan affinity.

Conclusion

BlMan5_8 is evolved for efficient deconstruction of soluble mannans, which is reflected by an exceptionally low K_m and the presence of an atypical low affinity CBM, which increases binding to specifically to soluble mannan while causing minimal decrease in catalytic efficiency as opposed to enzymes with canonical mannan binding modules. These features highlight fine tuning of catalytic and binding properties to support specialization towards a preferred substrate, which is likely to confer an advantage in the adaptation to competitive ecological niches.

Electronic supplementary material

The online version of this article (doi:10.1186/s12858-015-0055-4) contains supplementary material, which is available to authorized users.

Degradation of konjac glucomannan by Thermobifida fusca thermostable β-mannanase from yeast transformant

Native konjac glucomannan was used as the substrate for thermophilic actinomycetes, Thermobifida fusca BCRC19214, to produce β-mannanase. The β-mannanase was purified and five internal amino acid sequences were determined by LC-MS/MS. These sequences had high homology with the β-mannanase from T. fusca YX. The tfm gene which encoded the β-mannanase was cloned, sequenced and heterologous expressed in Yarrowia lipolytica P01 g expression system. Recombinant heterologous expression resulted in extracellular β-mannanase production at levels as high as 3.16 U/ml in the culture broth within 48 h cultivation. The recombinant β-mannanase from Y. lipolytica transformant had superior thermal property. The optimal temperature of the recombinant β-mannanase from Y. lipolytica transformant (pYLSC1-tfm) was 80°C. When native konjac glucomannan was incubated with the recombinant β-mannanase from Y. lipolytica transformant (pYLSC1-tfm) at 50°C, there was a fast decrease of viscosity happen during the initial phase of reaction. This viscosity reduction was accompanied by an increase of reducing sugars. The surface of konjac glucomannan film became smooth. After 24h of treatment, the DPw of native konjac glucomannan decreased from 6,435,139 to 3089.

The loop structure of Actinomycete glycoside hydrolase family 5 mannanases governs substrate recognition

Endo-β-1,4-mannanases from Streptomyces thermolilacinus (StMan) and Thermobifida fusca (TfMan) demonstrated different substrate specificities. StMan hydrolyzed galactosylmannooligosaccharide (GGM5; 6(III) ,6(IV) -α-d-galactosyl mannopentaose) to GGM3 and M2, whereas TfMan hydrolyzed GGM5 to GGM4 and M1. To determine the region involved in the substrate specificity, we constructed chimeric enzymes of StMan and TfMan and evaluated their substrate specificities. Moreover, the crystal structure of the catalytic domain of StMan (StMandC) and the complex structure of the inactive mutant StE273AdC with M6 were solved at resolutions of 1.60 and 1.50 Å, respectively. Structural comparisons of StMandC and the catalytic domain of TfMan lead to the identification of a subsite around -1 in StMandC that could accommodate a galactose branch. These findings demonstrate that the two loops (loop7 and loop8) are responsible for substrate recognition in GH5 actinomycete mannanases. In particular, Trp281 in loop7 of StMan, which is located in a narrow and deep cleft, plays an important role in its affinity toward linear substrates. Asp310 in loop8 of StMan specifically bound to the galactosyl unit in the -1 subsite.